1. Field of the Invention
This invention relates generally to the field of control electronics for a polyphase electric motor such as an induction or brushless DC motor and specifically to an integrated system for supplying polyphase motor drive signals from a DC source and for recharging the DC source from an external source of AC power.
2. Background Art
Widespread adoption of electric road vehicles has heretofore been hampered, in part, by the lack of suitable charging systems. Existing charging systems often rely on stationary off-board chargers with transformers and/or electronics that are too expensive and/or too heavy to be conveniently carried on board a vehicle. Alternatively, on-board charging systems with reduced cost and bulk may be employed, but at the cost of limited recharging power levels. For example, many of the on-board chargers that have been previously developed implement line frequency phase control with SCRs or triacs. Although these devices are cost effective, the draw power at unacceptably low power factors for wide spread high power applications.
Integrated motor drive and recharge systems have been developed so that certain of the components in the motor drive electronics are also used in a recharge mode as components of a switching regulator. Such an approach facilitates recharging at high power levels with substantial savings in the bulk and cost of the electronics package. For example, U.S. Pat. No. 4,920,475 issued Apr. 24, 1990 to Rippel discloses an integrated traction inverter and battery charger apparatus which integrates the components of a three-phase inverter with an AC line powered battery charger. A full wave rectifier bridge is included in the on-board electronics for conditioning the AC line power.
Another integrated motor drive and recharge system is disclosed in U.S Pat. No. 5,099,186 issued Mar. 24, 1992 to Rippel and this inventor. This system is designed for a dual motor application or for a single motor with dual sets of windings. In either case, the additional motor windings are advantageously employed to eliminate the need for an input bridge rectifier in the recharge mode.
A deficiency in the previously described integrated motor drive and recharge systems is high current in the ground wire connection to the power line. This results in a potential safety hazard if the ground wire is missing or improperly connected. This problem may also cause tripping of ground fault interruption (GFI) protected power outlets.